1. Technical Field
The present invention relates to a semiconductor device including an LD (lateral double-diffused) MOS field effect transistor having an impurity diffusion region extending in a lateral direction of a semiconductor substrate, and a method for manufacturing the semiconductor device.
2. Related Art
In the process that manufactures an LDMOS field effect transistor in a related art, in order to form an N-well, impurities such as phosphorus ions are implanted to a surface of a semiconductor substrate, and are then diffused at a high temperature for a long time. Thus, the impurity concentration distribution in the N-well is in the form of a Gaussian distribution in the depth direction. That is to say, the concentration of impurities in a region below a gate oxide film is largest at a position adjacent to the gate oxide film, and becomes lower in accordance with the depth from the surface of the semiconductor substrate.
Accordingly, the specific resistance is smallest at the surface of the semiconductor substrate, and is large in the internal portion (bulk region) of the semiconductor substrate. Thus, when a bias voltage is applied to a drain region and a gate electrode using a source region as a reference, almost all the current flows along the surface of the semiconductor substrate. As a result, the electric field is concentrated near an end portion of the drain region.
In particular, when a high bias voltage is applied, the amount of current that flows near the end portion of the drain region increases, so that impact ionization occurs. Thus, holes and electrons are rapidly generated, and the on-state breakdown voltage is significantly lowered. Accordingly, SOA (safe operating area) characteristics and the hot carrier resistance of the LDMOS field effect transistor significantly deteriorate.
In the LDMOS field effect transistor in the related art, in order to improve the SOA characteristics, it is necessary to increase the length of a drift region where a current flows in the N-well, that is, the length of a field oxide film, resulting in an increase in the element size. Accordingly, it is difficult to improve the SOA characteristics and the hot carrier resistance, while maintaining the current characteristics and the breakdown voltage characteristics.
As a related technique, JP-A-2001-352070 (paragraphs 0028-0031, FIG. 1) describes a semiconductor device that is intended to improve the ESD (electrostatic discharge) surge resistance. This semiconductor device includes a substrate having a semiconductor layer of a first conductivity type, a base region of a second conductivity type formed in a surface portion of the semiconductor layer, a source region of the first conductivity type formed in a surface portion of the base region, a drain region of the first conductivity type spaced away from the base region, in the surface portion of the semiconductor layer, a gate insulating film formed on a channel region that is the base region positioned between the source region and the drain region, a gate electrode formed on the gate insulating film, a source electrode, and a drain electrode.
The surface portion of the semiconductor substrate is further provided with a region of the first conductivity type disposed between the drain region and the base region, wherein the region of the first conductivity type has a concentration that is higher than that of the semiconductor layer and that becomes higher toward the drain region. If the region of the first conductivity type is disposed in this manner, a current value when the LDMOS field effect transistor enters a negative resistance region can be increased, and the ESD surge resistance can be improved.
However, even in the case where a region of the first conductivity type having a concentration that becomes higher toward the drain region is provided, most current between the drain region and the source region flows along the surface of the semiconductor substrate. Thus, it is not possible to significantly improve the situation in which the electric field is concentrated near the end portion of the drain region.